INSIGHT INTO SPATIO-TEMPORAL DISTRIBUTION OF WORLDWIDE SILURIAN CHERTS

A global survey of Silurian chert deposits at the epoch level suggests a Lower Silurian slowdown in silica accumulation. Conversely, renewed marine silica burial appears to characterize the Upper Silurian. Low latitude, deep-water environments seem to have been the dominant locus of silica deposition from the Middle Ordovician through the Silurian following a shift from a greater abundance in shallow-water environments during the Ordovician, and apparently kept this status at least until the beginning of the Devonian.

The relatively low abundance of chert deposits during the Lower Silurian could be linked to several factors including: 1) the late Ordovician extinction, 2) an early Lower Silurian rapid transgression and warming, and/or 3) unstable Lower Silurian climatic and oceanographic conditions driven by multiple glacial – interglacial intervals. The chert slowdown corresponds with and is very likely linked to a low in radiolarian diversity viewed as an extinction event, as well as hypothesized phosphate and reef gaps, a black shale-rich event, and diminished evaporitic facies. The relationship between all these parameters is still poorly understood, but is consistent with a warm world characterized by sluggish ocean circulation.

The predicted increase in Upper Silurian chert abundance occurs in a stable dominantly warm and dry climate. The increase matches a steady rise in radiogenic strontium probably related to the first phase of the Acadian orogeny, increasing radiolarian diversity, and a shift in the evaporite deposition toward higher latitudes. Biogenic chert abundance might be linked to high siliceous organism productivity related to augmented silica and nutrient availability in the marine realm through continental weathering, upwelling, volcanic and hydrothermal processes.